Tap conductors and 120% back feed rule

Status
Not open for further replies.

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Hi,

There is a 100 amp meter main combo panel with 2 breakers side by side. One is a 100 amp supplying a sub-panel, the other is for an A/C unit- may be a 50amp or so. If I were to tap on to the conductors on the load side 100 amp breaker supplying the sub-panel do I need to take into account the 120% busbar rating rule? I am trying to figure out if the busbar would ever see more than what it normally would with the solar backfeeding. Solar would go to loads in house first then out to utility, correct? Thanks for the help!
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
You do not need to apply the 120% rule to the busbar in the main panel. The 100A breaker is in effect a supply side connection at that location. As long as the busbar (and other components of the service) are sufficiently rated for the solar backfeed, you are okay.

If your AHJ insists on trying to apply the 120% rule, ask them what is the sum of the primary source breaker and the solar backfeed. (Hint: there is no primary source breaker protecting the busbar). I've had this conversation with several AHJs and it usually brings them around.

With that said...
You may not be able to tap that feeders as you plan if the conductors are not rated for the 100amp breaker plus the solar. The 2014 code treats this differently than previous codes, so you'd need to tell us what code cycle applies. One option might be to downsize the 100A breaker.

Also, hopefully this panel you speak of is actually rated more than 100A if it already has 150A of load breakers in it. Technically it only has to be rated for the calculated load, but the burden may be on you to either prove to your AHJ what the calculated load is, or downsize the breakers to meet the rating of the busbar.

One last thing: Check if that AC unit breaker can be switched out for a quad. A lot of panels are made this way. If so, you put a 50/solar quad in that space and you have an instant supply side connection, easy as pie.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Oh great thank you. Yeah I have not been to the site yet so will have to check those few things. So the conductors from the load side of the 100 amp feeder breaker would have to be rated for say 140 amps if I added 40 amps of solar? That does make sense to me.

The quad may be the way to go.

It is odd to me according to the pictures that were taken the panel is rated for 100 amps, however there is a 100 amp feeder breaker for the sub panel and a breaker for the A/C (couldn't tell the amperage, maybe a 50A). Ill need to see how it is wired.

Thanks again! Really appreciate it.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
... So the conductors from the load side of the 100 amp feeder breaker would have to be rated for say 140 amps if I added 40 amps of solar? That does make sense to me.

In the 2008 and 2011 codes you could make the argument that the conductor can fall under the 120% rule. So if you had 100 feeder and 40amps solar the feeder could be rated 117A. (117*120%= 140A). Not every AHJ might go for this.

The 2014 code does not allow this. But an option in this case could be to insert an overcurrent device to protect the feeder on the load side of the tap. The 2014 code specifically details these options: either rate the feeder for the sum of both sources or protect it with a load side device.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Okay, just so I understand you correctly- On the load side of the tap I could insert a 100 amp fused disconnect to protect those conductors? So then if the load was drawing more than 100 amps and the solar was supplying the rest that the load needed it would trip that breaker. So I could put a main breaker at the sub-panel rated for 100 amps with a lug kit if it does not have a main breaker already. If it does have a 100 amp main breaker already I am good, is this all correct? So if I did do this and add a load side breaker off the feeder tap could I backfeed up to 100 amps? Not that I will but just wondering. They are probably looking at 40 amps backfeed max.
 

Zee

Senior Member
Location
CA
"....you could make the argument that the [subpanel feeder] conductor can fall under the 120% rule. "

Jaggedben,
I agree with quad suggestion, and everything else, but I won't waste space writing my agreements. You ave stated them well enough.
I think you got this one wrong. Slightly.
Or right when you initially said 120% rule applies to tap conductors. Yes. But the follow-up requirement to using the 120% rule is "PV brkr opposite feed". There is no way to do that with a PV tap onto feeder conductors. Any tap will necessarily be before all loads and not opposite feed.

For 120% rule: His only solution would be breaker at opposite end of the busbar of the subpanel that the feeder feeds.
or
applying 100% rule: downsize feeder of 100A to 60A, if he adds 40A PV.........

Am i seeing this right?
 

Smart $

Esteemed Member
Location
Ohio
"....you could make the argument that the [subpanel feeder] conductor can fall under the 120% rule. "

Jaggedben,
....
I think you got this one wrong. Slightly.
Or right when you initially said 120% rule applies to tap conductors. Yes. But the follow-up requirement to using the 120% rule is "PV brkr opposite feed". There is no way to do that with a PV tap onto feeder conductors. Any tap will necessarily be before all loads and not opposite feed.

For 120% rule: His only solution would be breaker at opposite end of the busbar of the subpanel that the feeder feeds.
or
applying 100% rule: downsize feeder of 100A to 60A, if he adds 40A PV.........

Am i seeing this right?
JB is correct. The part you didn't quote "Not every AHJ might go for this" may be true, but it would be compliant under 2011.

For verification, 705.12(D)(2) applies... and because it is not connected through a subpanel breaker, 705.12(D)(7) does not apply. However, if the subpanel is MLO, its bus would have to be rated for at least 117A. No problem if subpanel has 100A MCB.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
"....you could make the argument that the [subpanel feeder] conductor can fall under the 120% rule. "

Jaggedben,
I agree with quad suggestion, and everything else, but I won't waste space writing my agreements. You ave stated them well enough.
I think you got this one wrong. Slightly.
Or right when you initially said 120% rule applies to tap conductors. Yes. But the follow-up requirement to using the 120% rule is "PV brkr opposite feed". There is no way to do that with a PV tap onto feeder conductors. Any tap will necessarily be before all loads and not opposite feed.

For 120% rule: His only solution would be breaker at opposite end of the busbar of the subpanel that the feeder feeds.
or
applying 100% rule: downsize feeder of 100A to 60A, if he adds 40A PV.........

Am i seeing this right?

So in the 2008 and 2011 codes, 705.12(D)(2) says, very simply, that the sum of breakers shall not exceed 120 percent of the busbar or conductor. No opposite feed rule or nothin'. Just basically says you can load up to 120%.

Then 705.12(D)(7) comes along and says that for a panelboard you have to make your inverter connection at the opposite end of the main feed. Nothing about conductors (or even busbars). This rule only refers to panelboards.

Hence the argument that these code cycles allow, simply put, loading a conductor up to 120% of its rating. A literal reading of the code pretty obviously allows it. Do I think this is safe? Well a couple things make me not too worried about it, at least in residential. One is that if the existing overcurrent device has never tripped from excessive load, then it's highly unlikely that it will trip after a load side tap. Of course someone else could add more loads downstream of the feeder, but in that case, if the feeder is overloaded with any kind frequency, at some point the main overcurrent will trip on a cloudy day. And this is likely to happen well before the conductor insulation deteriorates so much as to cause a fire. A third thing is that 120% is probably comfortably within the safety factors the NEC uses. This is a major confluence of independent circumstances. So I'm not losing any sleep over load-side taps that I've done up to 120% of feeder ratings. :p

Now the 2014 code cycle got rid of this silliness and requires feeders to be protected at their rating in one way or another, and the 120% rule now only applies to 'opposite fed' panelboards, period. This just makes more logical sense, all around. It does take away the load-side tap option for a lot of situations, or complicates it, but I've no objection to something which generally makes the code more logical, and potentially opens the door to raising that 120% for panelboards to something higher. It may also help with those AHJ's that weren't allowing load side taps, if you do them right. :cool:
 

Carultch

Senior Member
Location
Massachusetts
Now the 2014 code cycle got rid of this silliness and requires feeders to be protected at their rating in one way or another, and the 120% rule now only applies to 'opposite fed' panelboards, period. This just makes more logical sense, all around. It does take away the load-side tap option for a lot of situations, or complicates it, but I've no objection to something which generally makes the code more logical, and potentially opens the door to raising that 120% for panelboards to something higher. It may also help with those AHJ's that weren't allowing load side taps, if you do them right. :cool:

In the event that a 2014 rule helps me, but I have a project in a place that is behind on the NEC edition, do AHJ's typically allow taking advantage of future rules provided that all related rules are also followed?

In otherwords, I wouldn't want to install rapid shutdown in a place that is still in NEC2008. But at the same time, provisions in 2014 greatly simplify interconnection.

One reason I ask, is that I may end up doing this by accident, since I ordinarily work in a state where the NEC is adopted the instant the calendar year begins. Can't give a specific example, but there may be cases with a project in another state, where I specify something that I'm accustomed to in the present year, and unaware that it was ever different.
 
Last edited:

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I've never really tried to appeal to a later code to let me do something that wasn't allowed in a current code.

In one case, an AHJ brought it up and decided that for transformerless inverters we could follow the 2014 code while the 2011 was still in force. That actually caused me some severe problems because they put some other interpretations on top of that and started treating isolated inverters differently.

In a couple other cases, I appealed to a later version of the code as a way of clarifying the interpretation of the present code. But those issues didn't involve very substantive changes to the code.
 
You do not need to apply the 120% rule to the busbar in the main panel. The 100A breaker is in effect a supply side connection at that location. As long as the busbar (and other components of the service) are sufficiently rated for the solar backfeed, you are okay.

If your AHJ insists on trying to apply the 120% rule, ask them what is the sum of the primary source breaker and the solar backfeed. (Hint: there is no primary source breaker protecting the busbar). I've had this conversation with several AHJs and it usually brings them around.

With that said...
You may not be able to tap that feeders as you plan if the conductors are not rated for the 100amp breaker plus the solar. The 2014 code treats this differently than previous codes, so you'd need to tell us what code cycle applies. One option might be to downsize the 100A breaker.

Also, hopefully this panel you speak of is actually rated more than 100A if it already has 150A of load breakers in it. Technically it only has to be rated for the calculated load, but the burden may be on you to either prove to your AHJ what the calculated load is, or downsize the breakers to meet the rating of the busbar.

One last thing: Check if that AC unit breaker can be switched out for a quad. A lot of panels are made this way. If so, you put a 50/solar quad in that space and you have an instant supply side connection, easy as pie.

JB, you dont think that if that 100 amp breaker is supplying other non solar stuff that that muddys the water?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
JB, you dont think that if that 100 amp breaker is supplying other non solar stuff that that muddys the water?

Nope.

In the 2014 code this is rather explicit...

Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar containing loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed ...

If there is no overcurrent device protecting the busbar then you can't add it to the sum. :cool:

Combine that with the fact that the entire section this is under refers to connections on the 'load side of the service disconnecting means', and you have no way of applying the rule and no reason to apply the rule. In previous codes the language is a bit different but the logic is still the same.

The only things that I think are affected by 'other non-solar stuff' is whether you are allowed to add a set of service entrance conductors, and whether you have to count the breaker as one of the 'six handles' for service disconnecting means grouped in one location. (And since this panel is all fed by one set of service entrance conductors, you're only allowed six handles and it's moot in this case.)
 
Nope.

In the 2014 code this is rather explicit...



If there is no overcurrent device protecting the busbar then you can't add it to the sum. :cool:

Combine that with the fact that the entire section this is under refers to connections on the 'load side of the service disconnecting means', and you have no way of applying the rule and no reason to apply the rule. In previous codes the language is a bit different but the logic is still the same.

The only things that I think are affected by 'other non-solar stuff' is whether you are allowed to add a set of service entrance conductors, and whether you have to count the breaker as one of the 'six handles' for service disconnecting means grouped in one location. (And since this panel is all fed by one set of service entrance conductors, you're only allowed six handles and it's moot in this case.)

I agree with that, but I think we are talking about different issues. I am a bit confused by the exact set up in the OP, but my interpretation is he has 2 service disconnects in a MLO service rated panelboard. One of those feeds some "normal" stuff, and the other feeds some "normal" stuff AND also has PV tapped into the feeder between the breaker (service disconnect) and the "sub panel". You are calling that a supply side PV connection? Do I have all that correct?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
... You are calling that a supply side PV connection? Do I have all that correct?

One way to look at it as two connections in series: a load side connection (the tap) and a supply side connection (the service disconnect/feeder breaker in the main.) The load side connection has to follow the rules for load side connections, and the supply-side connection has to follow the rules for supply-side connections.

Another way to look at it is this: There's a service disconnect, and the connection is made on the load side of it. All of the rules for the load side apply only to stuff on the load side of the service disconnect. Whatever else exists on the supply side of the service disconnect is completely irrelevant, whether it be other parallel service disconnects, fire pump equipment, or anything else that Article 230 permits.

In my opinion the code needs some moderate reorganization to make clear that connections in series are allowed and should be treated this way. (There was some language in previous cycles about connections in series, but it was taken out of the 2014 because it referred to breaker sizes which are no longer relevant to any calculations.) The way the code is currently written just doesn't explicitly address series connections. A re-arrangement could also make it so that the rule limiting total sources to the ampacity of the service applies to all sources connected to the service, even if there are multiple sources connected in different ways.
 
Status
Not open for further replies.
Top